1. Field of the Invention
The present invention relates in general to an apparatus for polishing of materials, and relates in particular to a polishing apparatus having a revolving drum with a polishing pad mounted thereof for polishing an object such as a semiconductor wafer to a flat and mirror finish.
2. Description of the Related Art
High density integrated semiconductor devices of recent years require increasingly finer microcircuits, and the trend is for interline spacings also to be of steadily decreasing size. For optical lithography operations based on less than 0.5 micrometer interline spacing, the depth of focus is shallow and high precision in flatness is required on an object to be polished which has to be coincident with the focusing plane of the stepper. This requirement means that the wafer surface must be made extremely flat, and a first step in achieving such precision in flatness begins with proper surface preparation by polishing with a polishing apparatus.
FIG. 12 shows an example of a conventional type of polishing apparatus comprising a turntable 30, a top ring 31 which exerts a certain pressing pressure on the turntable 30 and an object 32 to be polished, such as a semiconductor wafer, which is disposed therebetween. A polishing pad 34 is disposed on the top surface of the turntable 30 against which the object is rotated to provide a flat and mirror polished surface. A discharge nozzle 33 is used to supply a polishing solution Q onto the polishing pad 34 which acts to retain the polishing solution Q. Usually, polishing is carried out by holding the object 32 below the top ring 31 so that the surface to be polished faces the polishing pad 34. In such an arrangement of the polishing apparatus, to provide a sufficient relative revolution speed between the surface to be polished and the polishing pad 34, the center axes of the top ring and the turntable are offset, i.e., not made concentric, so as to provide sufficient rotational displacement of the wafer relative to the polishing pad. This type of arrangement necessitates a configuration wherein the outer diameter of the turntable must be several times larger than that of the semiconductor wafer object. Further, it is also necessary to provide sufficient strength and rigidity, while maintaining the horizontal alignment of the turntable, to the turntable and the table frame so as to prevent the generation of harmful vibration of the rotating turntable, which vibration would interfere with the polishing process. For the polishing apparatus of the type shown in FIG. 12, these design requirements inevitably lead to the necessity of providing a large-space facility to accommodate a large polishing apparatus.
Furthermore, in a polishing apparatus of the type mentioned above in which the object is held on the top ring 31, the surface of the semiconductor wafer 32 being polished is pressed against the polishing pad 34 on the turntable, and it is not possible to view the condition of the wafer surface during the polishing operation. The result is that it is difficult to determine the amount of surface material (such as surface oxide film) removed or remaining on the wafer without disturbing the wafer in some way. Methods for determining the amount of film material removed or remaining have been presented, for example, in U.S. Pat. No. 5,089,716 which relates to moving a wafer away from a turntable during polishing. Another method, according to U.S. Pat. No. 5,196,353 is based on measuring variations in the temperature of the wafer to determine the elapsed time of polishing. However, such methods lead to a complex configuration of the apparatus, and in particular, although both methods permit some observation of the surface condition, the former relies on intermittent examination of the surface during polishing, while the latter relies on an indirect method based on the temperature variation in the wafer. In either case, it is difficult to obtain a satisfactory level of measurement precision.
On the other hand, Japanese Laid-open Patent Publication H2-269552 discloses a polishing apparatus having a revolving drum of cylindrical shape which revolves while polishing a wafer surface to be polished by contacting the wafer surface with a circumferential peripheral surface of the drum. The contact interface between the drum and the wafer surface is essentially along a line-shaped region on the surface to be polished, and a polishing solution is supplied to the contact region while some relative linear movement is provided along a path suitably directed with respect to the drum axis.
The polishing apparatus having such a revolving drum does not require a large diameter turntable as is required with the type of apparatus shown in FIG. 12, and therefore, the drum-type apparatus can be made compact and light weight. Also, an important advantage is that this type of apparatus enables an operator to observe the surface of the semiconductor object being polished, and to provide an accurate measure of the film thickness polished off or yet remaining on the wafer.
However, according to the method and the apparatus disclosed in the above-noted Japanese Laid-open Patent Publication H2-269552, polishing occurs only at the linear contact region between the revolving drum and the object. Therefore, when polishing a round object such as a semiconductor wafer, there is a tendency for the outer peripheral region of the wafer to be subjected to a higher pressing pressure than in the central region of the wafer, leading to higher rates of material removal in the peripheral region of the water, thus causing the so-called phenomenon of "peripheral degradation". Further, because polishing occurs at the linear contact region, it is difficult to apply an even pressure across the entire surface of the object. For example, of for some reason there is insufficient pressing pressure applied to the object during polishing of a local area thereof, there is a tendency to generate a wavey pattern on the polished surface, resulting in localized non-uniform polishing and potential generation of rejects.